The research goals of this proposal are to develop and test a new method for identifying the specific genetic loci that contribute to population variation in clinically relevant health risk factors. A unique sample of DNA specimens from approximately 250 11 year old twins and their parents are available to test this new research design. The samples will be typed for RFLP's known to be located at or near the loci which determine the currently recognized apolipoproteins. The dizygotic twins will be classified according to their identity by descent of the loci in question and compared with monozygotic pairs who are identical at all other loci as well. An overall analysis of the resulting data will permit accurate measurement of the proportion of the total genetics variation in serum cholesterol and its subfractions that is determined by variation at or near each of the apolipoprotein loci.